John Gurzinski/Getty Images(LAS VEGAS) -- President Obama on Tuesday urged Congress to move forward on a comprehensive bill to overhaul the nation's immigration system, saying the "time is now" to get it done.

During a speech in Las Vegas, the president laid out some "key markers" he believes must be a part of immigration reform, including a pathway to citizenship for many of the 11 million undocumented immigrants living in the U.S. He said if Congress does not act swiftly, he would send his own bill to Capitol Hill and "insist that they vote on it right away."

"We need Congress to act on a comprehensive approach that finally deals with the 11 million undocumented immigrants who are in this country right now," he said before a raucous crowd at Del Sol High School.

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While Obama gave an ultimatum to Congress, he sounded optimistic about lawmakers' efforts to craft their own proposals.

On Monday, a bipartisan group of senators called the "Gang of Eight" unveiled their own outline for comprehensive reform. It contains a pathway to citizenship for the undocumented, increased border security, and crackdowns on businesses that employ undocumented immigrants. It also brings much-needed skilled immigrant workers to the United States.

The Senate framework is "very much in line with the principles I've proposed and campaigned on for the last few years," Obama said. "So at this moment, it looks like there's a genuine desire to get this done soon, and that's very encouraging. But this time, action must follow."

Republicans reacted cautiously to Obama's speech. Sen. John McCain (R-Ariz.), a member of the Gang of Eight, noted that there are "some differences" between him and the president.

"I appreciate the president's support for our bipartisan effort on comprehensive immigration reform," he said in a statement. "The road ahead will be not be easy, but I am cautiously optimistic that working together, we can find common ground and move forward on this vitally important issue."

Passage of a comprehensive immigration reform bill in the House of Representatives could prove tougher. Yet House Speaker John Boehner (R-Ohio) did not rule out taking up a bill based on Obama's comments.

"There are a lot of ideas about how best to fix our broken immigration system," said Boehner spokesman Brendan Buck. "Any solution should be a bipartisan one, and we hope the president is careful not to drag the debate to the left and ultimately disrupt the difficult work that is ahead in the House and Senate."

The president's address stressed the similarities between his proposals and the Gang of Eight's, but there are indications that the two may differ in key areas.

For example, the senators' plan would allow undocumented immigrants to apply for legal status, but would not allow them to seek permanent legal residence until the border is deemed secure. In his speech, Obama did not specifically link border security to a pathway to citizenship.

"For comprehensive immigration reform to work, it must be clear from the outset that there is a pathway to citizenship," he said.

Obama also said that under his principles, a pathway to citizenship would include going through a background check, paying fines and back taxes, learning English, and going to the back of the line, meaning that they could not obtain citizenship until backlogs of legal immigrants are cleared.

"We all agree that these men and women should have to earn their way to citizenship," he said. "It won't be a quick process, but it will be a fair process."

White House officials made it clear before the speech that the president is withholding judgment on the Senate's plan until actual legislative language is drafted.

Obama did say that the government "needs to stay focused on enforcement" and any immigration reform bill should crack down on businesses that knowingly hire undocumented immigrants with stiffer fines and penalties.

According to a fact sheet released by the White House, the president also supports granting gays and lesbians the ability to "seek a visa on the basis of a permanent relationship with a same-sex partner" who is a U.S. citizen or permanent resident. Under current law, that provision only applies to heterosexual couples. The president did not mention the policy during his speech.

A temporary, guest-worker program also did not appear in Obama's speech or fact sheet. Business groups and Republicans have called for the implementation of such a program for agricultural industries and other companies that rely on low-skilled immigrant labor. But labor unions have been hesitant to support such programs without strong labor protections.

Obama has drawn criticism from lawmakers in both political parties and Latino groups for failing to bring up a comprehensive immigration reform bill in his first term along while piling up record deportation numbers. But the president was received warmly by the crowd inside Del Sol High School's gymnasium. They chanted "sí se puede!" during the speech.

The changing political landscape has sparked momentum for comprehensive immigration reform. In last November's election, seven in 10 Latino voters backed Obama over Republican Mitt Romney, helping him win battleground states like Nevada, where the president spoke Tuesday. But Obama said that immigration reform is about more than politics.

"Remember that this is not just a debate about policy. It's about people. It's about men and women and young people who want nothing more than the chance to earn their way into the American story. And throughout our history, that's only made our nation stronger," he said.